This invention relates generally to ultrasound probes such as used in medical diagnostic imaging systems, and more particularly the invention relates to an adapter for an ultrasound probe which facilitates imaging in the near field of the probe.
Ultrasound systems are now widely used in the medical profession for diagnostic imaging. There exist many systems which employ a phased array transducer probe for transmitting ultrasound signals and receiving reflections thereof from regions of a patient undergoing examination. The signals are steerable continuous or pulsed waves that can be processed to give depth and velocity information. The transmitted signals can be focused to various depths in the patient to be examined. However, a phased array probe has limitations in focus and resolution in the near field that can adversely affect imaging of surface regions such as the carotid artery in the neck, or other small anatomical parts, such as the breast and the testicle.
To provide better near field imaging, previous practices have been to use a material of similar sound velocity as tissue between the probe and the patient as a standoff, the material being 1 to 4 cm in thickness. A commonly accepted clinical practice is to fill a rubber glove with degassed water and place it between the probe and the patient. However, the filled glove tends to be slippery and difficult to position for optimal scanning. Another clinical practice has been to use rubber-like materials as a standoff, such as "fanny fat", which is a silicone gel used normally in floatation pads for supporting patients with superficial burns. The disadvantage of rubber-like materials in general is the high attenuation of soundwave, which limits the penetration in imaging and decreases the signal to noise ratio.
Adapters have been proposed for use with ultrasound transducer probes which can provide a standoff for the probe to facilitate imaging in the near field. One such support is disclosed in U.S. Pat. No. 4,296,753. This support utilizes a series of substantially coaxial, loop shaped elements which are axially arranged one behind the other within a fluid filled coupler. In an alternative embodiment the loop shaped elements are replaced by a spring. The loop shaped elements and spring facilitate the rocking of a probe for varying the direction of imaging. Another ultrasound probe device utilizing a fluid filled support is disclosed in European application No. 84102798, publication No. 0120410. The structure includes a plurality of pieces which are clamped together in a rigid arrangement. Because of the large number of piece parts in these designs, the cost of manufacturing is high. Other manufacturers have used metal or plastic structures to hold a thin plastic or elastomer bagfilled with a coupling material, or used rigid plastics as containers for the coupling fluid with acoustic "windows" made of thin plastic films on the container for the transmission of ultrasound through the container.
The above configurations have sub-optimal image quality when applied to a phased array. A phased array has multiple transmit-receive elements arranged in a regular spacing, similar to an optical grating. The net result of diffraction is the presence of side lobes (grating lobes) at an angle to the main beam. This implies that the transducer will be sensitive not only to objects at the main lobe of the beam, but also to objects at large angles to the beam. Since most rigid objects create strong echoes, the water path standoff consisting of rigid structures tends to increase the clutter in the image. The rigid structures also tend to be uncomfortable to the patient.